1. Field of the Invention
The present invention relates to a switching regulator type power supply circuit. More specifically, the present invention relates to such power supply circuit of a self-excited oscillation type.
2. Description of the Prior Art
A switching regulator type power supply of a self-excited oscillation type has been put into practical use, due to the fact that an external driver circuit, a driver transformer and the like can be dispensed with and hence a circuit configuration becomes simple. One type of such self-excited oscillation type power supply comprises a blocking oscillation type power supply, as proposed for example in German Pat. No. 2417628 (corresponding to British Pat. No. 1494259) with reference to FIG. 1.
Now the power supply circuit shown in FIG. 1 of the above referenced patent will be described to point out a technical problem to be solved by the present invention.
FIG. 1 is a schematic diagram showing such power supply circuit, which basically comprises an input rectifying portion 1, a blocking oscillator portion 2, a starter circuit 3, an output rectifying portion 4, and a control circuit portion 5.
In the following the above referenced patent will be described to the extent required for understanding of the present invention.
A switching transistor TR1 of an NPN type included in the blocking oscillator portion 2 is connected at the collector thereof to one terminal of an input winding N1 of a converter transformer T. The other terminal of the input winding N1 is connected to an input line L1 of a direct current voltage. The emitter of the switching transistor TR1 is connected to one end of a detecting winding N3 of a transformer T, i.e. a line L5 and the line L5 is a reference potential. A resistor R11 is connected to the line L5 so that a collector-emitter current flows through the resistor R11. The other end of the detecting winding N3 is connected through a line L4 and a diode D7 to a capacitor C5 and the other end of the capacitor C5 is connected to the reference potential, i.e. the line L5. The transformer T comprises a feedback winding N2 and one end a of the winding N2 is connected to the junction of a turning off capacitor C9 and the base of the switching transistor TR1, while the other end b of the winding N2 is connected to the reference potential, i.e. the line L5 through a series connection of a parallel connection of two diodes D10 and D11 coupled in the opposite poralities to each other and a resistor R12. The terminal b of the winding N2 is further connected through a diode D9 to the anode of a silicon controlled rectifier SC and further through a series connection of the diode D9 and the turning off capacitor C9 to the base of the switching transistor TR1. The cathode of the silicon controlled rectifier SC is connected to the input line L2 and through a parallel connection of a resistor R11 and a capacitor C7 to the reference potential line L5. The gate of the silicon controlled rectifier SC is connected to the junction of a voltage divider implemented by two resistors R9 and R10. One end of the voltage divider is connected to the reference potential line L5 and the other end of the voltage divider is connected through a diode D8 to one end a of the feedback winding N2. The capacitor C6 is connected in parallel with the voltage divider implemented by the resistors R10 and R11.
The collector of an error detecting transistor TR2 of a PNP type included in the control circuit portion 5 is connected through a resistor R7 to the junction B of two resistors R10 and R9 constituting a voltage divider. The base of the transistor TR2 is connected to the tap of a voltage divider implemented by the resistor R5, the variable resistor VR and the resistor R6 and the voltage divider is connected in parallel with the capacitor C5. A series connection of a zener diode D6 and a resistor R8 is connected in parallel with the capacitor C5 and the resistor R8 is connected between the emitter of the transistor TR2 and the reference potential line L5. The capacitor C4 is connected in parallel with the resistor R9.
The input line L3 is connected through a series connection of the diode D5, the resistor R4 and the capacitor C3 constituting the starting circuit portion 3 to the base of the switching transistor TR1. The resistor R3 is connected between the junction of the diode D5 and the resistor R4 and the input line L2.
The capacitor C7 is connected in parallel with the resistor R11. A series connection of the capacitor C10 and the resistor R13 is disposed between the collector and the emitter (the reference potential line L5) of the switching transistor TR1.
The converter transformer T has the output winding N4 having two output taps and having one end connected to the ground. The respective taps are connected through diodes D12 and D13 constituting the output rectifying portion 4 to the direct current output terminal. The capacitor C11 is connected in parallel with the diode D12 and the cathodes of the respective diodes D12 and D13 are connected through smoothing capacitors C12 and C13 to the ground.
The input lines L1 and L2 are connected directly and through a fuse F2, respectively, to the output of the bridge rectifier BD comprising four diodes D1 to D4 and constituting the input rectifying portion 1. The smoothing capacitor C2 and the resistor R2 are disposed in parallel with the bridge rectifier BD. Two inputs of the bridge rectifier BD are connected through the resistor R1 and directly, respectively, to the line filter LF. Two inputs of the line filter LF are connected through a power supply switch SW and through the fuse F1, respectively, to the alternating current input. The ends of the two coils of the line filter LF at the side of the bridge rectifier BD are coupled together by the capacitor C1.
The load circuit of the switching transistor TR1 comprises a series connection for the direct current input through the input winding N1 of the converter transformer T, the collector-emitter path and the resistor R11 via the input lines L1 and L2. The base control path of the switching transistor TR1 comprises a series connection of a parallel connection of the diodes D10 and D11 having the capacitor C8, the base-emitter path of the switching transistor TR1, the resistor R12 and the feedback winding N2.
The control circuit portion 5 having the error detecting transistor TR2 obtains a comparison voltage from the detecting winding N3 and the comparison voltage is rectified by the diode D7 and the rectified output is obtained from the capacitor C5 and thus from the junction C. The comparison voltage is applied through a voltage divider implemented by the resistor R5, the variable resistor VR and the resistor R6 to the base of the transistor TR2. The comparison voltage is compared by the transistor TR2 with the reference voltage generated by the zener diode D6. The collector current It of the error detecting transistor TR2 flows through the resistor R7 during the above described comparison.
A portion of the voltage generated across the capacitor C6 appears across the resistor R9, i.e. at the junction B in accordance with a voltage division ratio of the voltage divider implemented by the resistors R9 and R10. The capacitor C6 obtains a voltage from the feedback winding N2 through the diode D8.
The blocking oscillator portion 2 comprises the feedback winding N2, the diode D9 and the turning off capacitor C9. The capacitor C9 forms an operating voltage source of the silicon controlled rectifier SC so that the control path between the base and emitter of the switching transistor TR1 is blocked when the silicon controlled rectifier SC is rendered conductive.
The capacitor C6 is charged with the polarity corresponding to the polarity of the voltage generated across the resistor R9 and the voltage developed across the resistor R9 is applied to the gate of the silicon controlled rectifier SC as a bias voltage thereof, thereby to prevent the silicon controlled rectifier SC from being rendered conductive. The collector current It of the error detecting transistor TR2 flows through the resistor R9 and the current It forms a voltage in the direction opposite to the voltage developed across the capacitor C6. The voltage developed by the collector-emitter current applied to the resistor R11 is also directed to the opposite direction. The bias voltage applied to the gate of the silicon controlled rectifier SC is compensated by a large collector current of the transistor TR2 caused by too high an output voltage from the transformer T or by too large a collector-emitter current flowing through the resistor R11. If and when the compensating effect is sufficiently large enough to overcome the bias voltage, the silicon controlled rectifier SC would be rendered conductive. Accordingly, it is when the collector-emitter current of the switching transistor TR1 or the output voltage of the transformer T becomes too large that the above described compensating function is performed.
The starting circuit portion 3 is implemented by the input line L3, the diode D5, the capacitor C3 and the two resistors R3 and R4. The input line L3 is connected to the alternating current input and a pulse signal current Is having a repetition frequency rate identical to that of the alternating current input is supplied to the base of the switching transistor TR1 through the diode C5 and the RC elements R4 and C3.
In a normal operation the capacitor C6 and the turning off capacitor C9 are charged with the polarity shown during a non-conduction period of the switching transistor TR1. More specifically, in the non-conduction period of the switching transistor TR1 one end b of the feedback winding N2 becomes positive so that the capacitors C6 and C9 are charged with the currents Ir1 and Ir2, respectively, flowing through the paths shown. At that time the capacitor C9 is charged with a voltage lower than a predetermined voltage between the terminals b-a of the feedback winding N2 by a forward voltage drop (approximately 0.6 V) of the diode D9 included in the path of the current Ir2. The voltages across these capacitors C6 and C9 are also maintained during the conduction period of the switching transistor TR1.
On the other hand, in the conduction period of the switching transistor TR1 the capacitor C8 is charged to the forward voltage drop (approximately 0.6 V) of the diode D10 connected in parrallel therewith.
In the conduction period of the switching transistor TR1 a sum voltage of the voltage across the capacitor C9 and the voltage drop (approximately 0.6 V) between the base-emitter of the switching transistor TR1 is applied between the anode and cathode of the silicon controlled rectifier SC and thus between the anode and the reference potential line L5. When the voltage drop across the resistor R11 becomes lower than the gate voltage of the silicon controlled rectifier SC, i.e. the voltage at the point A becomes lower than the voltage at the point B, the silicon controlled rectifier SC is turned on. Accordingly, the turning off capacitor C9 is connected between the base and the emitter of the switching transistor TR1. Therefore, the switching transistor TR1 is reverse biased with the opposite polarity voltage shown of the capacitor C9, whereby the same is turned off. The silicon controlled rectifier SC as turned on need be turned off to be ready for the next conduction of the switching transistor TR1.
When the switching transistor TR1 is turned off, one end a of the feedback winding N2 is brought to a negative potential which is lower than the voltage at the point b by the above described predetermined voltage, when the voltage at the other terminal b is deemed as a reference. Accordingly, the point a becomes a negative potential lower with respect to the reference potential line L5 by a sum of the above described predetermined voltage and the voltage across the capacitor C8. Therefore, the base of the transistor TR1 becomes lower as compared with line L5 by a sum of the above described predetermined voltage and the voltage across the capacitor C8. On the other hand, a voltage slightly lower than the above described predetermined voltage is maintained in the turning off capacitor C9 during the conduction period as described previously. Accordingly, the cathode of the diode D9 and thus the anode of the silicon controlled rectifier SC comes to be supplied with a difference between the above described negative sum voltage and the voltage across the capacitor C9, with the result that the negative sum voltage becomes larger than the voltage across the capacitor C9 and the difference becomes nigative, whereby the silicon controlled rectifier SC is reverse biased. Thus, the silicon controlled rectifier SC is turned off. Accordingly, the switching transistor TR1 is turned on responsive to the input to the base thereof.
More specifically, a voltage developed at the point A due to a current Ii flowing through the collector and emitter on the occasion of conduction of the switching transistor TR1 and a voltage developed at the point B by a collector current It of the error detecting transistor TR2 and the first capacitor C6 for turing off are compared by means of the silicon controlled rectifier SC. If and when the voltage at the point A becomes lower than the voltage at the point B, the silicon controlled rectifier SC is rendered conductive, whereby the second capacitor C9 for turning off charged in the polarity as shown is connected between the base and emitter of the switching transistor TR1, whereby the said transistor TR1 is cut off. At that time the voltage at the point A is a negative voltage (deeming a line L5 as a reference potential) being increased by the above described current Ii as the time lapses, whereas the voltage at the point B is a negative voltage obtained by addition of the positive voltage determined by the magnitude of the above described current It and a constant negative voltage determined by the capacitor C6. Therefore, if and when the magnitude of the current voltage obtained at the point C from the detecting winding N3 changes, the magnitude of the negative voltage at the point B accordingly changes, with the result that the turn-off timing of the switching transistor TR1 is changed.
The FIG. 1 power supply circuit involves the following shortcomings. Firstly, since the FIG. 1 power supply circuit employs a silicon controlled rectifier SC, the same is inappropriate in the case where a constant voltage control operation of high efficiency is to be performed by selecting an oscillation frequency of the blocking oscillator portion 2 to be sufficiently high. The reason is that a silicon controlled rectifier of a low electric power commercially available is generally poor in a response rate.
Secondly, a discharging current of the first capacitor C6 for turning off the silicon controlled rectifier SC flows through resistors R9 and R10 and this current does not become constant, inasmuch as the collector current It of the error detecting transistor TR2 is not fixed. Therefore, it follows that the charging current Ir1 flowing from the feedback winding N2 to the above described capacitor C6 on the occasion of turning off of the switching transistor TR1 changes in accordance with the voltage at the point C. This means that the charging current Ir2 flowing to the second capacitor C9 for turning off and thus the voltage across said capacitor changes and accordingly it could happen that the switching transistor TR1 is not assuredly turned off.
Furthermore, although the starting current Is flowing to the blocking oscillator portion 2 flows from the line L3 of the input rectifying portion 1 through the path of the diode D5.fwdarw.the resistor R4.fwdarw.the capacitor C3.fwdarw.the base and emitter of the transistor TR1.fwdarw.the resistor R11.fwdarw.the line L2, on the occasion of starting a current flows through a path of the diode D5.fwdarw.the resistor R4.fwdarw.the capacitor C3.fwdarw.a.fwdarw.b of the feedback winding N2.fwdarw.the diode D11.fwdarw.the resistor R12.fwdarw.the resistor R11.fwdarw.the line L2 in addition to the above described path. The impedance of the latter mentioned path is approximately the same as that of the first mentioned path, inasmuch as the number of turns of the feedback winding N2 is small. Therefore, the starting circuit 3 must provide a current sufficiently larger than the required starting current and accordingly a power loss on the occasion of starting is increased.
Furthermore, the capacitor C6 and the diodes D8 and D11 are required in order to turn off the silicon controlled rectifier SC and the starting circuit 3 is implemented by the diode D5, the capacitor C3 and the resistors R3 and R4 (the resistor R3 is for discharging the capacitor C3), with the resultant disadvantage that the number of components is increased and reduction of the cost is hindered.